1. Field of the Invention
This invention relates generally to a method and apparatus for controlling the charging of a photoreceptor drum or belt in a duplicator, such as a xerographic copier and an electrophotographic printer and, more particularly, to a charge applying semiconductive tube or roller that rotates in unison and contact with the photoreceptor drum or belt.
2. Description of the Related Art
Duplicators, such as electrophotographic printers and Xerographic copiers have heretofore employed a corona charging means that typically uses a corona wire disposed longitudinally parallel to a photoreceptor drum and spaced a constant distance from the outer periphery of the drum. This corona wire typically has an approximately 5,000 volt charge of appropriate polarity impressed thereon to initiate ion propagation of the appropriate polarity to the photoreceptive surface of the drum. While this method is generally effective for distributing positive or negative charges on the surface of photoreceptive material, it suffers from several inherent shortcomings, which are more pronounced when the propagating ions are of negative polarity.
For example, since the corona wire is non-contacting and spaced a small distance from the photoreceptive drum, relatively large voltages must be impressed upon the corona wire to ensure uniform and complete deposition of positive or negative ions on the surface of the photoreceptive drum. Moreover, the presence of oxygen in the region between the corona wire and the photoreceptive drum results in the oxidation of ionized air, which produces significant amounts of ozone.
Office equipment employing this technology, such as desktop laser printers and copiers, are typically operated in an office environment that includes numerous people in close contact with the equipment. Ozone is believed by some persons to have a significant deleterious impact upon the respiratory system of people who are exposed to the ozone for prolonged periods of time. Previous laser printers have employed carbon impregnated cardboard filters in an attempt to counteract the dispersion of ozone. These filters, however, have been generally ineffective in significantly reducing ozone levels.
Further, the air space between the corona wire and the photoreceptive drum is a less efficient transmission medium of charged ions than, for example, semiconductive material. Accordingly, significantly high voltages are necessary to ensure a dense and uniform deposition of positive charges on the surface of the photoreceptive drum. This high voltage requirement necessitates the construction of large, expensive power supplies, which has a significant impact on the overall cost of relatively cost sensitive laser printers.
The present invention is directed to an apparatus for efficiently depositing a uniform electrical charge, positive or negative, on the surface of a photoreceptive drum of an electrophotographic printer at a significantly reduced charge applying voltage without reducing print quality and while reducing the production of ozone.